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Distinct functions of the 90 kDa heat-shock protein (hsp90) in oestrogen and mineralocorticosteroid receptor activity: effects of hsp90 deletion mutants.

ABSTRACT: Recent studies have confirmed that the 90 kDa heat-shock protein (hsp90) interacts both in vitro and in vivo with steroid receptors, encouraging further detailed physicochemical and functional analysis of its chaperone role. Thus, to explore the relationship between hsp90 and receptors, the baculovirus system was used to overexpress the chick hsp90 alpha (chsp90) along with the chick oestradiol receptor (cER) or the human mineralocorticosteroid receptor (hMR). These receptors were able to form 9 S complexes with chsp90, demonstrating the association of the co-expressed recombinant proteins. Three mutants of chsp90 (delta A, delta B and delta Z) have been created by deletion of the A (residues 221-290) and B (530-581) regions, rich in charged amino acids, and the Z (392-419) region, a putative leucine zipper. After co-expression, anti-receptor antibodies immunoprecipitated the cER or hMR complexed with the wild-type chsp90, the delta B or the delta Z mutant, but not with the delta A chsp90, indicating that deletion of the A region of chsp90 leads to a lack of interaction with these receptors. The hormone binding capacity of the cER was unaffected after its co-expression with each of the three mutants. In contrast, the hMR co-expressed with the delta B mutant failed to bind aldosterone, a finding confirmed in vivo by the absence of hormone-induced hMR nuclear translocation. Thus the B region is required for high-affinity ligand binding by the hMR. Our results suggest that the A region (but not the B or Z regions) is involved in binding of chsp90 to the cER and hMR, while the B region is essential for hormone binding by the hMR, consistent with a chaperone function for hsp90.

SUBMITTER: Binart N

PROVIDER: S-EPMC1136072 | biostudies-other | 1995 Nov

REPOSITORIES: biostudies-other

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Project description:Two isoforms of the 90-kDa heat shock protein, HSP90alpha and HSP90beta, are present in the cytosol of mammalian cells. Analysis by polyacrylamide gel electrophoresis under nondenaturing conditions (native PAGE) revealed that HSP90alpha predominantly exists as a homodimer and that HSP90beta is present mainly as a monomer [Minami, Kawasaki, Miyata, Suzuki and Yahara (1991) J. Biol. Chem. 266, 10099-10103]. However, only the dimeric form has been observed under other analytical conditions such as gradient centrifugation. In this study, therefore, we investigated native forms of HSP90 by use of immunochemical techniques with isoform-specific monoclonal antibodies recently developed in our laboratory. Glycerol gradient centrifugation at the physiological salt concentration as well as native PAGE analysis of rat liver cytosol revealed oligomeric forms of HSP90alpha sedimenting at 8-10S as predominant ones. On the other hand, the glycerol gradient centrifugation revealed multiple forms of HSP90beta oligomers sedimenting at 6-12S. All of the HSP90beta oligomers, however, migrated at 100-kDa monomer and 190-kDa dimer positions on native PAGE. A novel two-dimensional double native PAGE revealed that the entity was converted from the HSP90beta dimer to monomers during the electrophoresis. The same PAGE further revealed that the HSP90alpha oligomer also dissociated into dimers during the electrophoresis. Full-length form of bacterially-expressed human HSP90alpha migrated as dimers, but a considerable amount did not penetrate into the gel under native PAGE conditions, indicating the existence of oligomeric forms. Electrophoretic studies of deletion mutants of HSP90 demonstrated that the C-terminal 200 amino acids were capable of forming oligomers. Taken together, we conclude that both of the HSP90 isoforms predominantly exist as oligomeric forms in the cytosol even under unstressed conditions but that they artificially dissociate into smaller forms when subjected to native PAGE.

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Distinct functions of the 90 kDa heat-shock protein (hsp90) in oestrogen and mineralocorticosteroid receptor activity: effects of hsp90 deletion mutants.

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